Marine Sgt. Andrew Tahmooressi says he crossed into Mexico with firearms illegal in that country by mistake. Mexican authorities aren't convinced. Critics blame US authorities for doing too little to secure his release.

Attorney Fernando Benitez and Jill Tahmooressi, mother of former US Marine Andrew Tahmooressi, talk to media outside a federal court in Tijuana on Wednesday. Sergeant Tahmooressi has been held for more than three months in a Mexican jail on weapons charges.

Jorge Duenes/Reuters

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WASHINGTON — The Marine sergeant and Afghanistan war veteran jailed in Mexico since April 1 on charges he entered Mexico with illegal firearms will remain behind bars south of the border at least until August, following a court hearing in Tijuana Wednesday.

In the meantime, the case of Andrew Tahmooressi – who claims he made a wrong turn and accidentally arrived at a border crossing into Mexico near San Ysidro, Calif., with three loaded weapons and more than 400 rounds of ammunition – will remain a thorn in the side of US-Mexico relations and a cause célèbre on social media, with veterans groups, and particularly among US conservatives.

From the US side of the border, the case looks crystal clear: Sergeant Tahmooressi, who had recently moved to California from Florida to get treatment for post-traumatic stress disorder, simply became disoriented on the night of March 31 and turned his pickup into a poorly marked lane along the border that took him straight to a border crossing into Mexico. The weapons he had with him were registered and legal in the US.

More than 70 members of Congress have signed a letter to the Tijuana judge hearing Tahmooressi’s case, stating “we stand behind Andrew and believe the specific circumstances of this case require Andrew to be returned to the US immediately.”

Not so fast, comes the retort from Mexico. Mexican officials say Tahmooressi’s case is being conducted under strict adherence to Mexican law, which calls for a judge to decide guilt or innocence based on the facts and on Mexican law. In Mexico, the weapons and ammunition Tahmooressi was transporting are only legally carried by the Mexican military.

“It should be underscored that the Federal Law on Firearms and Explosives is of fundamental importance in the actions Mexico takes to prevent and combat violence in its territory, particularly in the border region,” Mexican Attorney General Jesús Murillo Karam said in a statement on the case last month.

Mexico is extremely sensitive about the entry of firearms from the US, with studies – including one by the US Government Accountability Office – showing that the vast majority of weapons used by drug gangs and other criminals in Mexico originate north of the border.

On Wednesday, federal Judge Victor Octavio Luna Escobedo heard Tahmooressi’s testimony in a closed hearing and continued the case to Aug. 4.

Legal experts on both sides of the border say the outcome – Tahmooressi faces 10 years or more in prison if convicted of carrying firearms into Mexico – is likely to turn on Judge Luna Escobedo’s determination of Tahmooressi’s intent.

Many in the US appear to find it absurd to consider the Marine anything other than a well-intentioned newcomer to the border region confused by poor road signage and unaware of Mexican firearms laws. Members of Congress pointed out in their letter that Tahmooressi called 911 from the border crossing seeking advice on how to get back to the US.

But many on the other side see room for doubt on the matter of intentions. Those include the Mexican embassy in Washington, which put out a 12-page fact sheet suggesting that other facts make the case murkier than first meets the eye.

For one thing, the Mexican side says, the claim of confusion over border crossings has to be weighed against the reality that Tahmooressi had crossed the border to Mexico on at least three other occasions – including one visit earlier on the same day he was detained, during which he rented a hotel room. (The Mexican embassy helpfully offers a photo of Tahmooressi’s Tijuana hotel receipt in its fact-sheet packet).

They also say that multiple signs warn drivers crossing the border that weapons and ammunition are illegal in Mexico.

And Mexican officials say that Americans who have been quick to criticize Mexico for holding Tahmooressi since April should consider that his case has been delayed by the defendant himself after he fired his own legal counsel.

Such arguments hardly register on the US side, however, where social media sites and mostly conservative news shows blast both governments involved: the Mexican government for, as they say, acting more like an enemy than any ally; and the US government – and, in particular, the Obama administration – for doing little to return a US citizen and war veteran home.

The Fox News Channel, which has closely followed Tahmooressi’s detention since April, aired a one-hour special on the case as part of its Fourth of July coverage.

Veterans organizations have also taken up the cause. The Veterans of Foreign Wars wrote a letter to President Obama demanding he give “as much urgency” to securing the release of Tahmooressi as he did to freeing longtime Taliban captive Sgt. Bowe Bergdahl. Last month the VFW called for a boycott of Mexico, saying that if “political pressure” on both governments wouldn’t work, maybe “the power of the purse” could.

Some analysts of US-Mexico relations say it would be hypocritical of the US – which has long chided its southern neighbor for a weak judicial system and frequent failings in the rule of law – to now press Mexico to overlook its laws in this case because it involves a US citizen.

But some in the US turn that argument around and say it is Mexico that can be charged with hypocrisy.

Rep. Duncan Hunter (R) of California, who has been pressing for Tahmooressi’s release since April, was incensed last month when the Mexican government apologized after a Mexican Army helicopter flying along the border fired on US border patrol agents, saying it was a mistake.

“It’s ironic that Mexico says it acted accidentally in this case and they ask we accept an apology,” Representative Hunter said, “when they refuse to acknowledge an authentic mistake on Andrew’s part.”